In a mobile telephone system the communications between a mobile station (MS) and a mobile telephone exchange (MTX) is performed via routes known as channels. The signal traffic in the channels consist of signalling and the actual sound, i.e. speech traffic. In this context the speech is indented to mean the sound to be transmitted to and reproduced on the receiver side. The signalling part of the traffic is used for exchanging messages between the MTX (known also as Mobile Switching Center, MSC) and the mobile stations. The signalling handles e.g. the setting up and clearing calls, handover, updating, ordering change of transmitting power etc. The signalling transfers also various alarms and in addition, handles the remote control of the base station between the MTX and the MS. The signalling equipment consists in general of a modem (or modems) and coding equipment. In addition, the MTX functions for equalizing group delay distortion.
In analog mobile systems the analog part consists of the air or radio interface between the mobile station and the base station. So as to enable signalling traffic in the system, the binary position in the signalling is transferred as a frequency shifted signal in accordance with Fast Frequency Shift Keying (FFSK) method. As an example, with a data rate of 1200 bit/s, logical "one" is represented by one period of frequency of 1200 Hz, and logical "zero" by 1.5 period of frequency of 1800 Hz. The transition between a "one" and "zero" occurs at the zero transition points of the signal. All signalling is done with aid of frames, which are always of the same length and consist at the present systems of e.g. 166 bits. During reception, the first 15 bits may be used for bit synchronization. The transfer of the signal frames is done over the whole transmission channel.
In the present analog mobile communications systems the signalling and the voice transmission is transmitted on the same traffic channel. Therefore it is necessary to split the signal on the traffic channel before it arrives to the ear of the receiving party of the call. The splitting procedure can also be defined as muting of speech path during the signalling.
In one prior art solution the splitting is arranged by using an ETC board (Exchange Terminal Circuit board) of the MTX which is arranged to split the signal as response to a demand by the software of the exchange. However, due to a delay in the splitting caused by the software of the exchange a part of the signalling comes to the ear, and, instead of signalling, silence is transmitted to the PSTN (Public Switched Telephone Network). The delay caused by the ETC circuit is estimated to be up to 20 ms.